•We summarize the recent progresses for the enhancement of efficiency of UCNPs.•The challenges and opportunities for the highly efficient UCNPs were discussed.•Advantages and shortcomings of UCNPs were systematic compared with QDs & dyes.•The recent progresses of UCNPs based applications were summarized.

SummaryOver the past decade, high-quality lanthanide doped upconverting nanoparticles (UCNPs) have been successfully synthesized with the rapid development of nanotechnology and are becoming more prominent in wide application fields, especially in biological sciences. Compared with the traditionally used biological labels such as organic dyes and quantum dots (QDs), upconversion nanomaterials have many advantages, including higher chemical stability, lower toxicity, and higher signal-to-noise ratio. However, the low upconversion efficiency of the lanthanide UCNPs is still the most serious limitation for their applications. Improvements are still needed to optimize upconversion optical properties for further applications. In this review, we summarize the recent progresses for the enhancement of upconversion efficiency of UCNPs and discuss the challenges and opportunities to realize the highly efficient upconversion nanomaterials by systematic comparison of the advantages and shortcomings between UCNPs and QDs & organic dyes from various aspects, such as the luminescence mechanism, multicolor emission, luminescent stability, efficiency, ultra-small nanocrystals fabrication and so on. Furthermore, this review describes the recent progresses of UCNPs based applications in multiplexed encoding, guest delivery and release system, photodynamic therapy (PDT), solar cell, photocatalysis and so on. We also detail the major barriers that currently prevent UCNPs from mainstream applications.